It is known that an electric power system is planned and designed for normal operation with respect to determined dimensioning criteria as regards system variables. The nominal frequency of the electric power system is one such variable. The mains frequency deviation of the electric power system from the nominal frequency is a measure of the power balance in the electric power system.
When a disturbance arises in an electric power system or the electric power system is abnormally loaded, an impermissible condition arises in the electric power system beyond said dimensioning criteria. For the purpose of cancelling this impermissible condition, protective functions are built into the electric power system, for example implemented by utilizing protective devices such as relay protection devices at defined connection points, nodes, in the electric power system. Said relay protection devices initiate disconnection of parts of the load or the production in the electric power system, in dependence on instantaneous values of said system quantities in said nodes.
Power balancing by means of underfrequency-controlled load disconnection is a known method that is used in many electric power systems as a protective function for avoiding a breakdown if the mains frequency falls below the setting for the process. The power balancing is carried out according to the prior art, for example by means of frequency relay protection devices. Load disconnection is performed with a mutual order between all the frequency relay protection devices included in the power system according to a predetermined connection plan. For a general description of load disconnection and for selection of a connection plan, reference is made to (P. Kundur): Power System Engineering Series, Power System Stability and Control, 1993, EPRI Editors, pages 623-626.
A frequency relay protection device may utilize as tripping criterion the mains frequency of the electric power system; another alternative is to utilize the derivative of the frequency.
Within a geographical region with a fixed nominal frequency, for example 50 Hz or 60 Hz, the mains frequency is allowed to vary within a fixed frequency interval comprising the nominal frequency. When there is a need for power balancing in such an electric power system, the load disconnection occurs in a fixed number of steps according to a connection plan, determined by the network operating management for said electric power system, at different frequency levels and time lags in different nodes, with the frequency level and the time lag in dependence on the relevant frequency level based on current regulations for the region in question. For each step, a function with direct load disconnection and/or a time-lag function with a definite time-lag load disconnection may be carried out.
Within Nordel, where the nominal frequency is 50 Hz, the mains frequency is normally allowed to vary between 49.9 Hz and 50.1 Hz. When a need for power balancing arises in an electric power system within Nordel, the load disconnection takes place as mentioned above in a fixed number of steps according to a connection plan, determined by the network operating management for said electric power system, at different frequency levels and time lags in different nodes, with the frequency level and the time lag in dependence on the relevant frequency level based on Nordel's regulations. Typical values for the time lag are 0.15 s for direct load disconnection and 20 s for definite time-lag disconnection.
A frequency relay protection device that has a time lag for definite time lag set initially when the system is put into service may be reset with a new time lag, for example after a load disconnection has been initiated by said frequency relay protection device, whereby said resetting with a new time lag is made with a view to optimizing the setting of said frequency relay protection device in the electric power system in question. This method changes the valid connection plan since the frequency relay protection devices are mutually related according to this connection plan, which results in resetting of also the other frequency relay protection devices in the electric power system.
Production disconnection is another known method that is used in many electric power systems as a protective function for avoiding a breakdown in the electric power system. Production disconnection is overfrequency-controlled and is carried out with a frequency relay protection device of the same type as described above for underfrequency control but the frequency and time parameters of which have been given other values.
However, it may happen that such load disconnection or production disconnection is performed in a node, connection point, which is not suitable for the present condition of the power system, which leads to said disconnection aggravating the impermissible condition, hence activating additional protective functions, which further aggravates the impermissible condition and causes a breakdown to occur in the electric power system. To make possible continued operation, it is thus of great importance at which connection points, nodes, in the electric power system that the load disconnection is made.
Power balancing by means of underfrequency-controlled load disconnection or overfrequency-controlled production disconnection takes place as described above according to the prior art by means of disconnection of load or production. Admittedly, said disconnection is performed in dependence on the instantaneous mains frequency, but according to a connection plan with a predetermined selection of mutual order between the connection points, nodes, of the electric power system. However, this may result in disconnection of a node, connection point, that is unsuitable for the current condition of the power system.